The present invention relates to a process for dyeing asphalt and/or bitumen with inorganic granulated pigments.
The processing of pigments in powder form is increasingly considered to be critical in terms of dust development and dosing capability. On the one hand, during processing a reduction in danger and nuisance to humans and the environment is to be ensured, and on the other hand the observation of quality criteria leads to more stringent requirements in terms of colour effect. Colour effect is substantially influenced by adherence to the highest possible level of accuracy in the formulation, i.e. improved dosing leads to greater quality constancy. On the other hand, powder pigments frequently can only be dosed inaccurately on account of the adhesion forces.
Dust nuisance and poor dosing capability are of particular significance in processes which, on account of environmental regulations and the cost situation, only permit the use of cheap, low-dust pigments which are capable of being dosed. This is the case in the dyeing of asphalt-bitumen for the construction of roads and paths.
Here mainly cheap iron oxide red compounds are used in a low dosage level. However, other inorganic pigments, such as for example iron oxide yellow, titanium dioxide or chrome oxide green, are also used.
In the past there has been no lack of attempts to eliminate dust nuisance. Thus it is standard procedure for pigments for the dyeing of asphalt to be filled into plastic bags and for the total quantity of pigment, together with the packaging, to be introduced into the hot asphalt. In this case, melting point of the packaging material must be distinctly lower than the processing temperature to enable the packaging material to dissolve sufficiently rapidly. Furthermore, in the case of discontinuous mixing apparatus, adequate time must be allowed to ensure a complete and thorough dispersion of the batch of pigment.
More modern apparatus operate continuously. Here a batch-wise addition of the pigment powder is no longer possible, and for reasons of quality continuous dosing of the pigment must be provided.
Processes for the production of dust-free granulated pigments have been repeatedly described, for example for the dyeing of building materials in U.S. Pat. No. 4,946,505 and DE-A 3 918 694. The granulated pigments in question consist of micro-granules which, although dust-free and easily dosed, cannot be used for dyeing asphalt.
Granulated materials have also been described in association with asphalt. JP-A-74 005 210 has described a coating containing bitumen which is composed of metal- or metal oxide pigments, floats on the surface, and thus covers the black colour of the bitumen. In the case of only superficial dyeing, chalking and wear lead to a poorer colour effect in the case of long term use.
U.S. Pat. No. 4,852,870 has disclosed oil-coated granulated colouring pigments for the surfaces of sports areas. However, this constitutes a use outside that of typical asphalt, as the material in question is of a discrete nature and can be moved in the form of a layer on a base using a rake.
JP-A 57 205 447 describes the use of red sludge from bauxite production in the asphalt industry. Here the red sludge is converted by "flushing" into a red-brown slurry of iron oxide in oil which is then mixed with asphalt.
EP-A 191 278 describes aqueous, moist granulated iron oxide for the dyeing of building materials and bitumen compounds only from the standpoint of dust avoidance.
None of these processes combines the production and use of low-dust, free-flowing, easily dosed granulated materials with good dispersion properties for the uniform dyeing of asphalt.
Thus the object of this invention was to make available a process for dyeing asphalt which does not involve the disadvantages of the described prior art.
A process which fulfils these requirements has now been discovered.